drm/tegra: dsi: Refactor in preparation for command mode

For command mode panels, the DSI controller needs to be enabled and
configured so that panel drivers can send commands prior to the video
stream being enabled.

Move code from the monolithic output enable/disable functions into
smaller, reusable units to allow more fine-grained control over the
controller state.

Signed-off-by: Thierry Reding <treding@nvidia.com>